THE CENTURY'S PROGRESS IN PHYSICS 



refraction that govern light. But, for that matter, the 

 experiments that had established the mechanical equiva- 

 lent of heat hardly left room for doubt as to the imma- 

 teriality of this "imponderable." Doubters had, indeed, 

 expressed scepticism as to the validity of Joule's exper- 

 iments, but the further researches, experimental and 

 mathematical, of such workers as Thomson (Lord Kel- 

 vin), Rankine, and Tyndall in Great Britain, of Helm- 

 holtz and Clausius in Germany, and of Regnault in 

 France, dealing with various manifestations of heat, 

 placed the evidence beyond the reach of criticism. 



Out of these studies, just at the middle of the cen- 

 tury, to which the experiments of Mayer and Joule had 

 led, grew the new science of thermo-dynamics. Out of 

 them also grew in the mind of one of the investigators 

 a new generalization, only second in importance to the 

 doctrine of conservation itself. Professor William 

 Thomson (Lord Kelvin) in his studies in thermo-dynam- 

 ics was early impressed with the fact that whereas all 

 the molar motion developed through labor or gravity 

 could be converted into heat, the process is not fully re- 

 versible. Heat can, indeed, be converted into molar 

 motion or work, but in the process a certain amount of 

 the heat is radiated into space and lost. The same 

 thing happens whenever any other form of energy is 

 converted into molar motion. Indeed, every transmuta- 

 tion of energy, of whatever character, seems compli- 

 cated by a tendency to develop heat, part of which is 

 lost. This observation led Professor Thomson to his 

 doctrine of the dissipation of energy, which he formu- 

 lated before the Royal Society of Edinburgh in 1852, 

 and published also in the Philosophical Magazine the 

 same year, the title borne being, "On a Universal Ten- 



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